Tracing Fault Tolerance

A fault may cause a process to behave abnormally, and a fault hypothesis divides such abnormal behaviour into exceptional and catastrophic behaviours. The set of normal and exceptional behaviours can be considered the set of acceptable behaviours. In this report traces, or communication histories, are used to denote the behaviour of a process. The semantic function ℋ[P] defines the set of possible communication sequences that can be observed up to any point in an execution of process P. A fault hypothesis is defined as a predicate representing a reflexive relation between the normal and acceptable histories of a process. Such relations enable one to abstract from the precise nature of a fault and to focus on the exceptional behaviour it causes. For a fault hypothesis χ the construct (P≀χ) indicates execution of process P under the assumption of χ. Then, the set ℋ[(P≀χ)] is the set of acceptable histories of P with respect to χ.